Photolysis of chlorite by solar light: An overlooked mitigation pathway for chlorite and micropollutants

Chlorite (ClO₂⁻) is an undesirable toxic byproduct commonly produced in the chlorine dioxide and ultraviolet/chlorine dioxide oxidation processes. Various methods have been developed to remove ClO₂⁻ but require additional chemicals or energy input. In this study, an overlooked mitigation pathway of ClO₂⁻ by solar light photolysis with a bonus for simultaneous removal of micropollutant co-present was reported. ClO₂⁻ could be efficiently decomposed to chloride (Cl⁻) and chlorate by simulated solar light (SSL) at water-relevant pHs with Cl⁻ yield up to 65% at neutral pH. Multiple reactive species including hydroxyl radical (•OH), ozone (O₃), chloride radical (Cl•), and chlorine oxide radical (ClO•) were generated in the SSL/ClO₂⁻ system with the steady-state concentrations following the order of O₃ (≈ 0.8 μΜ) > ClO• (≈ 4.4 × 10⁻⁶ μΜ)> •OH (≈ 1.1 × 10⁻⁷ μΜ)> Cl• (≈ 6.8 × 10⁻⁸ μΜ) at neutral pH under investigated condition. Bezafibrate (BZF) as well as the selected six other micropollutants was efficiently degraded by the SSL/ClO₂⁻ system with pseudofirst-order rate constants ranging from 0.057 to 0.21 min⁻¹ at pH 7.0, while most of them were negligibly degraded by SSL or ClO₂⁻ treatment alone. Kinetic modeling of BZF degradation by SSL/ClO₂⁻ at pHs 6.0 – 8.0 suggested that •OH contributed the most, followed by Cl•, O₃, and ClO•. The presence of water background components (i.e., humic acid, bicarbonate, and chloride) exhibited negative effects on BZF degradation by the SSL/ClO₂⁻ system, mainly due to their competitive scavenging of reactive species therein. The mitigation of ClO₂⁻ and BZF under photolysis by natural solar light or in realistic waters was also confirmed. This study discovered an overlooked natural mitigation pathway for ClO₂⁻ and micropollutants, which has significant implications for understanding their fate in natural environments.